Protein Quaternary Structure: Symmetry Patterns

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Abstract Protein molecules can assemble into larger, multisubunit oligomers that possess unique quaternary structures and biological properties. The properties of the oligomeric protein can be quite different from those of the individual subunits. The noncovalent protein–protein interactions giving rise to oligomers often result in symmetric complexes containing identical monomers in identical environments. The symmetry properties of these complexes are described, in particular with respect to the types of subunit–subunit interactions giving rise to the oligomers. Because the symmetry of the macromolecular assemblies is based on the symmetric interactions between the subunits, understanding the symmetry provides further insight into the functioning of the oligomers. Specific examples presented include: dimers and heptamers with cyclic symmetry, tetrameric and octameric oligomers with dihedral symmetry, multienzyme complexes with tetrahedral and dodecahedral structures, and pilin helical oligomers. Key Concepts: Many proteins form large complexes made up of multiple copies of identical subunits. Analysis of the quaternary structures of protein complexes in terms of symmetry can determine how equivalent the subunits are, in function as well as structure. Subunits in dimeric proteins related by twofold rotation axes interact using the same functional groups on each subunit, that is, isologously. Protein complexes showing cyclic and helical symmetries have subunits that do not interact with the same functional groups on each subunit; they interact heterologously. Large protein complexes can take on the symmetries shown by the Platonic solids. Helical symmetry is found in large rod‐like structures such as pili. Protein complexes showing point group symmetry are constructed using all of the available subunit–subunit binding surfaces and are ‘closed’ to addition of further subunits. Helical structures containing identical subunits have unoccupied binding surfaces at the termini of the helix that can bind additional subunits and grow into unlimited polymers.